In evaluating a new petroleum discovery, formation fluid samples are acquired for analysis. Such samples are typically acquired by open-hole wireline fluid sampling (WFS) and brought to the surface for analysis. Accordingly, as currently practiced, mud filtrate contamination of a sample is typically not measured until after the sample is brought to the surface. If excessive mud filtrate contamination is detected after the sample is brought to the surface, the sample is deemed invalid and is discarded. Even if the sample is suitable for use, time has usually been wasted in extra flushing of the sampling tool when an earlier sample would have been good enough.
There are four situations involving oil based mud (OBM) filtrate or water based mud (WBM) filtrate contamination of formation fluid samples. These are OBM filtrate contamination of oil samples; WBM filtrate contamination of oil samples; OBM filtrate contamination of connate water samples; and WBM filtrate contamination of connate water samples. The last situation, WBM filtrate contamination of connate water samples, is not addressed in the prior art. Thus, there is an unfulfilled need for a method and apparatus for measuring downhole water-based mud (WBM) filtrate concentration in a sample of connate water before the sample is brought to the surface.